Indeed! I finally found some time to make progress:
btrfs replace start -B /dev/mmcblk0p2 /dev/sda1 /
btrfs filesystem resize max /
reboot
moved the file system to the new ssd. I also managed to set up a btrfs RAID 1 on my two 1TB HDD’s connected via USB. After creating two empty partitions on /dev/sdb and /dev/sdc (I used Cockpit for this) I did:
mkfs.btrfs -L "Nas" /dev/sdb -f
mkfs.btrfs /dev/sdc -f
mkdir /mnt/nas
mount /dev/sdb /mnt/nas
cd /mnt/nas
btrfs subvolume create backup
cd
umount /dev/sdb
mount -o compress=zlib,subvol=backup,autodefrag,noatime /dev/sdb /mnt/nas
umount /dev/sdc
btrfs device add -f /dev/sdc /mnt/nas
btrfs filesystem balance start -dconvert=raid1 -mconvert=raid1 /mnt/nas
btrfs filesystem show /mnt/nas
With the help of northerncoder.ca.
Then set up Samba to back up my Windows 10 Box. Then backed up my Linux box with rsync. So far everything works as expected.